Method of making a waterproof transformer seal



Oct. 23, 1962 J. E. BUDOVEC EIAL 3,

METHOD OF MAKING A WATERPROOF TRANSFORMER SEAL Filed March 31, 1958 2Sheets-Sheet 1 FIE- E fiz Z/IZ forts Josep/z Z Zadodec 3,059,283 METHODOF MAKING A WATERPROOF TRANSFORMER SEAL Joseph E. Budovec, Hollywood,and Franklin J. Connolly, Cicero, Ill., assignors to Jefferson ElectricCompany, a corporation of Delaware Filed Mar. 31, 1958, Ser. No. 724,9433 Claims. (Cl. 18-59) This invention relates to a method of making awaterproof transformer seal.

In out door installations, it is customary to mount a transformer in acontainer which may be in the form of either a box-like structure or'acylindrical tank, plus a cover. The cover is provided with a pipe or anipple, and the transformer lead are brought out through this pipe ornipple. The transformer proper is embedded within the tank or othercasing by means of a suitable potting compound. Although the pottingcompound is water resistant to a certain extent, its physicalcharacteristics are such that it does not form a bond with the sheathingof the transformer leads. Thus, small crevices may open up which permitthe introduction of water down into the windings. Therefore, means arecustomarily provided to seal the leads to the nipple.

Such sealing means as heretofore utilized may comprise a packing glandwhich is threaded onto the nipple, or it may comprise a boot which fitsover the nipple and has a portion which may embrace the leads eitherseparately or collectively, but in either event it is customary to tapethe leads to the boot in order to prevent the entry of moisture into theinterior of the casing. According to such prior art methods, it issometimes customary to fill the nipple also with the potting compound.However, the potting compound merely serves to fill up the space anddoes not act as a seal to any greater extent than if the nipple were notfilled. Still another expedient has been to place within the nipple arubber washer having a separate hole for each lead, then to draw theleads through the holes, and then to pour asphalt in the nipple abovethe washer.

It is an object of the present invention to seal a transformer coverwithout the use of auxiliary mechanical means, such as a packing gland,a boot, or a perforated washer.

It is another object to seal a transformer cover with an improved sealwhich has improved mechanical properties to the end that the transformeras a whole may be lifted or pulled around by the leads without incurringthe danger of opening up any cracks or crevices which would destroy thewaterproof characteristics of the transformer.

According to the present-invention, the foregoing objects are achieved,in part, by providing a transformer cover in which the nipple is filledwtih a reaction setting epoxy resin, the leads being embedded in theresin. The mechanical properties of epoxy resin are such that it forms abond with the neoprene wire jacketing and also with the metal surfaceswhich not only prevents the seepage of water along the interfaces, butwhich is so tenacious that it permits a 50 to 100 pound transformer tobe lifted by the leads without opening up any cracks or crevices.Furthermore, the epoxy resin material in itself is a good insulator withthe result that there will be no electrical strain between any highvoltage leads and the transformer casing or nipple.

Still another object is to provide an improved method of making a sealof the type described which improved method greatly reduces the laborrequired. According United States Patent to. this aspect of myinvention, the leads and the cover are first assembled separately, andthe seal effected by filling the nipple with the epoxy resin. Then theleads are connected to the transformer proper, and the transformer putin the casing at the same time that the cover is placed on the same.Then the casing may be filled with the potting compound through asuitable opening in the cover, which opening is later sealed by a metaldisc soldered in place.

Still another object is to provide an improved method of filling thenipple to effect the desired seal in such a manner that the epoxy resinis prevented from running out of the nipple as it is being filled.

One of the problems encountered in effecting a preassembled seal of theprecast type is that of plugging up the nipple during the casting orfilling operation.

A still further object of my invention is to provide an improved methodand means for partially obstructing or plugging the nipple during thisfilling or casting step, which means can be readily removed after theepoxy resin has set, either before or after heat curing thereof.

Other objects, features and advantages of this invention will becomeapparent as the description proceeds.

Referring now to the drawings in which like reference numerals designatelike parts:

FIG. 1 is an elevation of a transformer manufactured in accordance withthis invention;

FIG. 2 is a section of the cover showing the preassembled seal;

FIG. 3 is a plan section taken along line 3-3 of FIG. 2;

FIG. 4 is an elevation of the pattern used in fabricating the strippablemold;

FIG. 5 is a view similar to FIG. 4 but showing the formation of themold;

FIG. 6 is a view showing the completed mold and after it has beenstripped from the pattern and cut;

FIG. 7 is a sectional view of the inverted cover with the strippablemold applied, and showing the relationship of these parts to the leads,as well as the manner in which the strippable mold serves to positionthe leads and to maintain them out of contact with the metal of thecover;

FIG. 8 is a view similar to FIG. 6, but showing a modi fied type ofmold; and

FIG. 9 is a plan section taken along line 9--9 of FIG. 8.

With reference now to FIGS. 1 to 3, the reference numeral 10 designatesa transformer such as a street lighting transformer or ballast of a typewhich is now in common use for mercury vapor street lights. Thetransformer comprises the core and windings, not shown, which aredisposed within a casing or tank 11 which is provided with a cover 12.The cover may be suitably sealed to the tank to provide a Weatherproofstructure, as by brazing, welding or rolling.

The cover is provided with a nipple 13 which may be threaded asindicated at 14, if desired. The leads 15 for the windings extendthrough the nipple 13. The leads are provided with the customarysheathing 16 which may be of neoprene for example.

Ihe casing 11 and its associated parts are of any suitable material,such as galvanized sheet steel for the casing 11 and cover 12, and ashort length of galvanized steel pipe for the nipple 13, the latterbeing welded to the cover.

Disposed within the nipple is a cast plug 17 which provides the seal,the leads 15 being imbedded Within the plug 17. The plug is of amaterial that forms a good mechanical bond at the interface 18 betweenthe plug and the metal of the nipple, and at the interface 19 of theplug with the sheathing 16.

The cover is also provided with an opening 20 through which pottingcompound, not shown, may be introduced into the container as previouslyindicated. The rim 21 of the opening 20 may be depressed as shown toreceive a suitable disk 22, shown in broken lines. After the containerhas been filled, the disk is soldered to the depressed rim 21, and thisforms the final step in the closing and sealing of the transformer.

According to the present invention, the plug 17 is of epoxy resin whichis cast in situ, thus forming an exceedingly strong bond at theinterfaces 18 and 19, so strong, in fact, that a 50 lb. transformer maybe lifted by the leads without incurring any danger of either opening upthe bond or cracking the plug itself. As will be pointed outhereinafter, the material of the plug is actually a two componentthixotropic epoxy base resin.

The method of making the seal, which is to say, the method of castingthe plug in situ, will now be described with reference to FIG. 7.

FIG. 7 shows the cover 12 in an inverted position and placed over thenipple 13 is a strippable mold 30. The leads 15 extend through thestrippable mold 30 and the nipple 13. The elements here shown representthe relationship of the parts during an intermediate stage ofmanufacture, which stage, is to be followed by pouring of the epoxyresin into the interior of the nipple 13.

The strippable mold 30 comprises a large diameter base 31 which tightlysurrounds the nipple 13, a narrow neck portion 32, and an intermediatecylindrical portion 33. A connecting portion 34 accommodates the offsetbetween portions 31 and 33. A diagonally disposed connecting portion 35connects portions 32 and 33.

The interior of the nipple 13 cooperates with the strippable mold 31 toprovide a chamber 36, of which the narrow neck portion 32 constitutes arestricted outlet. The outlet is sufficiently large as to permit theleads 15 to be easily drawn through narrow neck portions but it issufficiently small as to constitute an obstruction to the free flow of arelatively viscous fluid therethrough. The presence of the leads 15 inthe neck also constitute an obstruction which further restricts thistype of fluid flow. Thus it is possible to plug the nipple during thefilling operation without resorting to the use of mechanical sealingdevices.

Although the filling operation outlined below in its broader aspects isapplicable to various types of viscous quick setting resins, we preferto utilize a thixotropic resin to effect plugging. The properties of thethixotropic epoxy base resin are such that the material, prior tosetting, is a free flowing fluid when the various elements thereof aresubject to pouring or flowing stress; however, if the motion of theparticles of the fluid is substantially arrested, the substanceimmediately jells, with the result that the substance loses its fluidcharacteristics.

The above mentioned restriction serves to arrest the flowing motion ofthe substance to an extent sufiicient to cause it to jell at a pointwithin or just below the connecting portion 35. Thus the continuouspassageway through the nipple 13 and the mold 30 is obstructedsufficiently so that the material above this point will jell as soon asit comes to rest within the interior of the nipple.

Thus, it is possible to cast the plug 17 in situ without the use ofauxiliary mechanical plugging means. The resin material is poured orsuitably injected into the chamber 36 until the entire space is filled,even providing a meniscus 37, as shown in FIG. 2.

After the filling operation, as above described, the cover assembly ispermitted to set for the required period, which, in the present example,is about six hours. At the end of the curing time the resin has been setup in its final form, after which the mold 30 may be stripped from thenipple and the leads 15. Any excess resin which extends below theconnecting portion 35 may be trimmed off for the purpose of appearanceto provide a finished appearing bushing as shown in FIG. 2.

The offset between the portions 31 and 33 is provided to improve theappearance as indicated in FIG. 2.

An important feature of the foregoing process is that the neck portion32 serves to center the leads 15 within the chamber 36, and consequentlywithin the plug 17, so that the possibility of having the leads contactthe metal of the nipple can be eliminated, particularly if the upperends of the leads are held in a centered position during the fillingstep.

The fabrication of the strippable mold 30 is shown in FIGS. 4, 5 and 6in which the reference numeral 40 designates a pattern which may beformed of aluminum. The pattern is dipped into a suitable thermoplasticresin to provide a coating 41 as shown in FIG. 5, which coating, up oncooling, constitutes the strippable mold 30, except for the bottomportion 42. The coating 41 is removed from the pattern 40, the patternsurfaces being suitably tapered to facilitate easy removal, and thebottom portion 42 is cut off to provide the finished mold 30, shown inFIG. 6.

The resin into which the pattern 40 is dipped is preferably a moltencellulose acetate butyrate resin having a melting point of 350 F. Theadvantage of this type of resin is that no mold release compound isrequired in order to render the same strippable either from the patternor from the nipple; furthermore, the resin sets at room temperature andcan be hastened by cooling in water.

The nipple stripping operation can be speeded up by slitting the mold 30with a sharp knife and then the rejected mold can be thrown back intothe pot in which the pattern 40 is dipped.

In actual practice, the dipping operation is mechanically effected, anda plurality of patterns 40 are mounted in a single holder so that thesupply of strippable molds is sufficient to permit continuity ofsuccessive filling operations.

Thixotropic properties are imparted to the resin by in corporating intoone of the components of the epoxy resin, a colloidal silica in theamount of V of 1% by weight (referred to the weight of both components),although up to 2% by weight can be used if desired. A suitable type ofamorphous type silica is sold under the trademark Cab-O-Sil, by John L.Cabot Company, Boston, Mass.

The epoxy resins that may be used in our process include the variousfluid polyaryl ethylene oxide resins obtained by reacting a polyhydricphenol such as a bis'phenol with epichlorhydrin or alphadichlorhydrin asdescribed in Castan U.S. Patents 2,324,483 and 2,444,333. Particularlysatisfactory results have been obtained with the epoxy resin sold byRubber and Asbestos Corporation of Bloomfield, New Jersey, under thetrademark Bondmaster formulation No. M638. The components are mixed justprior to the filling operation. V H

A modified strippable mold 43 is shown in FIGS. 8 and 9 in which themold 43 is provided with a base portion 44 and an intermediate portion45 corresponding to portions 31 and 33 of the mold 30. However insteadof a single neck portion 32, a double neck portion 46 is provided so asto further restrict the cross section of the outlet.

To summarize the operation which has been described above in detail, thecover assembly is first fabricated by casting the plug 17 in situ aboutthe leads 15. Then the inner ends of the leads are connected to thetransformer, not shown, the latter placed in the casing 11, and thecover 12 fitted to the casing, and sealed thereto. Then the casing isfilled with potting compound through the opening 20 and the disk 22 issoldered in place. Thus an entirely waterproof structure is providedwithout the use of mechanical seals and without incurring thesubstantial labor cost incident to taping the leads to the nipple or tothe boot or other sealing device.

Furthermore, the cast plug 17 provides a mechanical bond with theneoprene lead sheathing 16 and with the metallic surface of the nipplewhich Will Withstand a pull of up to 500 pounds, a force sufficient topermit the handling of a 50 to 100 pound transformer by its leads.

The present method of casting the plug 17 in situ permits the provisionof a plug which has a portion 38 which projects beyond the nipple 17,thus eliminating any concavity which would collect water. Thus splittingof the nipple due to freezing of the water cannot occur. Furthermore,the present method permits any desired shape to be imparted to theprojecting portion, so that a finished appearance can be obtained.

Although only a preferred embodiment of our invention has been shown anddescribed herein, it will be understood that various modifications andchanges-may be made in the construction shown without departing from thespirit of our invention as pointed out in the appended claims.

We claim:

1. The method of sealing leads to the nipple of a transformer coverwithout the use of auxiliary mechanical nipple plugging means comprisingthe steps of placing over the nipple a mold having a narrow neck portionlocated beyond the outer end of the nipple to provide a chamber having arestricted outlet, drawing the leads through said narrow neck portionand said nipple, maintaining said cover in an inverted position with thenipple extending downwardly, and with said restricted outlet at thebottom of said chamber, filling said chamber with a thixotropic reactionsetting resin, curing said thixotropic resin, and

stripping said mold from said nipple, the diiference between thecross-sectional area of said restricted outlet and the cross-sectionalarea of said leads being sufliciently small with respect to theviscosity of said resin as to cause said resin to gel substantiallyimmediately, the reaction setting of said resin occurring while saidresin is in a gelled condition.

2. The method as claimed in claim 1 in which said thixotropic resin isan epoxy base resin.

3. The method as claimed in claim 1 in which said mold is of a celluloseacetate butyrate resin.

References Cited in the file of this patent UNITED STATES PATENTS2,414,525 Hill et al J an. 21, 1947 2,511,436 Kauth June 13, 19502,701,392 Eich Feb. 8, 1955 2,713,700 Fisher July 26, 1955 2,756,270McDonnell July 24, 1956 2,763,708 Brennan Sept. 18, 1956 2,786,007 ChewMar. 19, 1957 2,892,013 Gornberg June 23, 1959 2,908,744 Bollmeier Oct.13, 1959 2,947,799 Finck Aug. 2, 1960 FOREIGN PATENTS 792,164 GreatBritain Mar. 19, 1958

